1. Field of the Invention
This invention relates to paper feed rollers which have a high coefficient of friction and are stable against environmental changes and are effective for plain paper copiers and facsimile machines.
2. Prior Art
Image forming apparatus as typified by electrophotographic copying machines and laser printers include a paper feed section for picking up discrete sheets of paper from a paper container via a najar roller and a paper separating mechanism and then delivering each sheet of paper toward an image transfer section via a conveyor roller arrangement along a paper feed path. In general image forming apparatus, the najar roller for picking up sheets of paper is disposed relative to the paper container such as a paper cassette or tray. The paper separating mechanism cooperates with the najar roller to separate the sheets of paper for allowing only a single sheet of paper to advance to the paper feed path.
Several systems are known for the paper separating mechanism. A paper separating mechanism of the friction retard system includes a paper feed roller and a retard member which may be either a fixedly secured pad or a retard roller.
The paper separating mechanism of the type equipped with a retard roller uses a means for holding the roller in a fixed state and causing the roller to rotate at predetermined timing, for preventing local wear of the retard roller surface.
Also known is a paper separating mechanism of the feed and reverse roller (FRR) system which uses a feed roller and a reverse roller adapted to rotate to the reverse of the paper feed direction. When sheets of paper enter between the feed and reverse rollers, only a sheet of paper on the feed roller side is delivered and a sheet or sheets of paper on the reverse roller side are fed back to the paper container. Paper separating mechanisms of the FRR system are disclosed in for example, Japanese Patent Application Kokai (JP-A) Nos. 57841/1984 and 192036/1991. In the mechanisms of these patent applications, a drive shaft for driving the reverse roller in a direction opposite to the paper feed direction is supported via a friction member which is biased by a coil spring for controlling the force of the friction member applied to the reverse roller upon reverse rotation.
In the conventional paper separating mechanisms mentioned above, both the feed roller and the retard roller are made of rubber or similar material in order that these rollers may have appropriate coefficients of friction relative to paper. Especially, the retard roller is made of natural rubber or polynorbornene to ensure that it pushes back a sheet of paper.
The rollers of these materials, however, suffer from wear and degradation due to low wear resistance and poor stability like the materials used in conventional najar rollers. Especially the retard roller is rubbed by a sheet of paper to be fed forward at the instant when an entrained sheet of paper is fed back to the tray. The retard roller thus undergoes substantial wear and degradation.
In addition, the roller members are sensitive to environmental changes and aging and substantially lose their coefficient of friction at low temperatures, failing to ensure normal paper feed operation.
EPDM and similar materials are sometimes used, but they also tend to lose their coefficient of friction at low temperatures. Then the paper separating mechanism fails to provide paper separating ability, allowing plural paper feed.